Honeycomb Candy (Cinder Toffee) Explained
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Honeycomb candy, also called cinder toffee or hokey pokey depending on where you grew up, is one of the few candies where the reaction happens almost entirely after the sugar comes off the heat. A spoonful of baking soda stirred into molten caramel sets off a burst of foaming that locks a network of air bubbles into place, giving the finished candy its light, porous, glass-like structure.
Before that reaction can happen, the sugar syrup needs to be cooked to the hard-crack stage, the final and hottest stage of candy cooking. The Exploratorium’s Science of Candy guide puts this range at 300 to 310 degrees Fahrenheit, where the syrup has almost no water left in it and a drop of it in cold water turns into hard, brittle threads. Cooking to this stage matters because honeycomb needs to set into a rigid, glassy structure once it cools; syrup pulled off the heat any earlier still has too much moisture to trap the bubbles rigidly, and it will collapse into a chewy, sticky mass instead of shattering cleanly.
The chemistry starts the moment baking soda hits that hot syrup. Sodium bicarbonate is not stable at high heat: as one chemistry lab manual puts it, “when heated, baking soda…decomposes, yielding sodium carbonate, water, and carbon dioxide,” and in cooking that breakdown happens almost instantly at candy-making temperatures. The carbon dioxide gas released by that reaction has nowhere to go in syrup this thick and viscous, so instead of escaping, it gets trapped as countless tiny bubbles throughout the mixture. As the honeycomb cools and hardens within seconds, those bubbles are locked in place permanently, which is what gives the finished candy its foamy, porous interior instead of a solid caramel one.
Because the reaction is instantaneous and the syrup is starting to set the moment it happens, working quickly is not optional. Baking soda should be sifted in and whisked for only a few seconds, just long enough to distribute it evenly, before the foaming mixture is poured out onto a lined baking sheet and left completely alone. Stirring for too long, scraping the pan, or trying to spread the honeycomb out with a spatula will knock much of that trapped gas back out, leaving a denser, flatter candy with far less of the signature bubbled texture. The honeycomb should be poured and then not touched again until it has fully cooled and hardened.
Once set, honeycomb is one of the more fragile candies to store, since it is essentially a sugar glass riddled with air pockets that readily pull in moisture from the surrounding air. Any humidity will soften and eventually dissolve the surface, turning a crisp piece sticky and dull within hours if it is left uncovered. Broken into pieces and kept in an airtight container, ideally with a layer of parchment between pieces so they do not fuse together, honeycomb keeps its crunch for one to two weeks at room temperature, though it is at its best eaten within the first few days.
Honeycomb’s brittle, bubbly structure also makes it a useful building block rather than only a finished candy in its own right. Broken pieces dipped in tempered chocolate turn into a familiar candy-bar style treat, while crushed honeycomb folded into ice cream or scattered over cakes and brownies adds crunch and a faint caramel bitterness that plain sugar candy does not have. Because the sugar glass dissolves quickly once it touches moisture, honeycomb used as a topping is generally best added just before serving rather than stored that way for later, and any coating in chocolate should be allowed to set fully before the pieces are stacked or boxed together.
A few practical adjustments make the process easier for a first attempt. Using a pot noticeably larger than the volume of syrup gives the foaming reaction room to expand upward without overflowing the sides, since a vigorous baking soda reaction can triple the mixture’s volume in an instant. A silicone spatula or balloon whisk folds in the baking soda more evenly than a metal spoon, and lining the baking sheet with parchment rather than simply greasing it makes lifting the finished, still-fragile sheet of candy much less likely to crack it apart before it can be broken into pieces on purpose.